1. Field of the Invention
This invention relates to carrier supported inks which contain sublimable dyes and to a process and apparatus for printing fabric or plastic film substrates with such carrier supported inks to render the images printed on the substrates abrasion and chemical resistant.
The invention is particularly suitable for use in preparing printed fabric or plastic film labels which can be subjected to corrosive laundering conditions or highly abrasive conditions without effacing their legibility.
2. Description of the Prior Art
Prior to the present invention, printed fabric labels were made using a variety of well-known techniques, such as screen printing, offset lithography printing, dyeing, flexographic printing, and in-plant printing.
Screen printing, also known as silk screen, employs a porous stencil mounted on a screen, in which the nonprinting areas are protected by the stencil. Printing is done on a mechanized press by feeding the cloth under the screen, applying ink with a paint-like consistency to the screen, and spreading and forcing it through the fine mesh openings with a squeegee.
In the offset lithography method, the image and non-image areas are essentially on the same plane of the surface of a thin metal plate, the definition between them being maintained chemically. The ink is picked up by the hydrophobic areas on the plate but is not picked up by the hydrophilic areas. The image is then transferred to an offset rubber roll, then from the roll to the fabric sheet.
Flexographic printing is a form of rotary letterpress using flexible rubber plates and fast-drying fluid inks. The rubber plates utilize the relief method for image creation, where the image area is raised above the non-image areas. Ink rollers touch only the top surface of the raised area. The surrounding, non-printing, areas are lower and do not receive ink. The inked image is transferred directly to the cloth. Dyeing can be achieved by using dyestuffs rather than pigmented inks in any of the printing processes described above. The use of dyes, however, requires additional after treatments to fix the dye in the fabric.
All of these methods lend themselves to use on large scale commercial printing equipment in which large sheets or webs of fabric are printed, and then cut or slit into strips for fabric labels. These labels are suitable for use in garments for the purpose of decoration, identification, advertising, wash and care instructions, size, price, as well as other purposes.
However, there are economic disadvantages to labels produced by these large commercial methods. Purchasers of labels must buy large minimum quantities of a particular type of label to maximize economies of scale. Therefore, due to the large number of possible combinations of colors, size, fabric content, etc. for a particular garment and minimum order requirements, a large inventory of labels must be maintained to ensure that each garment receives the proper labelling which is required. In addition to the obvious costs of maintaining an excess inventory, there are costs of label obsolescence. Moreover, an insufficient inventory results in improperly labeled garments which many times cannot be used until they are labeled properly. This often results in manufacturing or garment processing facilities which must shut down until proper labels can be obtained.
These disadvantages have led to the widespread use of in-plant printing systems which allow label users to print labels, as required, in direct response to their manufacturing needs. The main advantage of in-plant printing systems is that label changes can be made quickly, eliminating the need for large inventories of pre-printed labels, with subsequent economic advantages. Early methods of in-plant printing systems utilized small versions of flexographic or similar printing systems using a fluid or paste ink. An example of the type of system which can be used to print fabric labels with an indelible ink is disclosed in U.S. Pat. No. 3,733,212. In this system, a fabric label made of polyester or of a polyester/cotton blend and having a thermoplastic adhesive layer is printed with a liquid ink which contains a water dispersed dye capable of indelibly dyeing the fibers of the fabric when heated to an elevated temperature.
Although these in-plant printing systems are still in use today, they have in large part been supplanted by hot stamping in-plant printing technology which utilizes carrier supported ink. A carrier supported ink, often referred to as a hot stamping foil, comprises a dry ink coating on one side of a carrier sheet. Examples of carrier supported inks can be found in U.S. Pat. Nos. 3,441,425 and 4,251,276. The printed image is created when metal relief type or characters on a printing plate are brought into intimate contact with the uncoated side of the carrier. The coated side is simultaneously brought into intimate contact with the appropriate fabric under pressure and/or heat. Upon removal of the printing element or plate from the carrier, the carrier supported ink is pulled away from the fabric, leaving the image on the fabric where the top areas of the raised printing elements came into contact with the carrier. The advantages of this method over in-plant ink printing systems are that there is no ink to spill or clean up, the print opacity and contrast is superior, and the print quality is more controlled since the ink coating is of uniform thickness. All of the above methods provide textile and garment manufacturers with labels of high print quality and durability under most requirements of washability and dry cleaning.
However, one segment of the garment industry, industrial uniform manufacturers and laundry services, has been provided with high quality, durable printed fabric labels only from large scale commercial printing houses, and not from in-plant printing systems. Industrial uniforms are subjected to extremely corrosive and abrasive laundering conditions which destroy the legibility of ordinary fabric labels. Therefore, preprinted dyed labels, where the image is actually fixed into the fabric of the label, and preprinted labels with cured or thermoplastic overcoatings are required to maintain legibility. Industrial uniform manufacturers require print legibility for advertising purposes and to allow the garment user to maintain size integrity of his garments after each industrial laundering. If the garment size on the label becomes illegible, the garment cannot be classified properly according to size. Industrial uniform services have observed that unless label legibility is maintained, a high percentage of uniforms are lost, stolen, or misplaced during the laundering process.
Thus, the industrial garment industry is forced into using only preprinted labels during the manufacture and laundering processes, despite their large inventory requirements and obsolescence costs because the most practical, economic, and versatile labels printed with carrier supported ink in-plant printing technology do not survive repeated industrial launderings.
Textiles have been printed with designs for many years using a technique referred to as transfer printing. In an early version of this technique known as wet transfer printing, a paper carrier sheet was printed with a design using a gravure printing technique with a wide variety of dye classes, such as vat, acid and direct dyes, and acetate dispersed dyestuffs. The printing inks contained thermoplastic components. When the printed paper was brought in contact with the fabric, transfer of the print from the paper to the fabric was effected by applying heat and pressure. The prints on the fabric were then treated with steam to set the dyestuffs followed by washing to remove thickeners. Such aftertreatments are those employed in conventional techniques for dyeing fabrics. An example of this type of transfer printing is shown in U.S. Pat. No. 2,911,280.
Another technique for transfer printing is referred to as thermacrome or melt transfer printing. In this technique, a design is printed onto a carrier paper with a thermoplastic resin containing a pigment. To effect transfer of the printed design to a fabric, the printed paper is heated in contact with the fabric whereupon the resin melts and is partially transferred to the fabric substrate. Such prints do not require an aftertreatment. Variations of this technique are often referred to as decalcomania. U.S. Pat. Nos. 4,037,008 and 4,038,123 are exemplary.
A third type of transfer printing technique is known as dry transfer, sublimation transfer or vapor phase transfer. Using this technique, a heat resistant carrier such as paper is printed with inks containing sublimable dyestuffs. The printed paper is then placed in contact with the fabric surface. When heat and pressure are applied to the back of the paper, the dyes sublime from the ink on the paper, diffuse across the air gap to the surface of the fibers in the fabric, condense on the fibers and then diffuse into the interiors of the fibers. Since not all of the dye sublimes during one application, the transfer paper can often be used a number of times to transfer the same design. This third type of transfer printing has become the most widely accepted of these methods for printing fabrics with designs. Numerous examples of this type of transfer printing can be found in U.S. Pat. Nos. 3,363,557, 3,484,342, 3,707,346, 3,813,216, 3,888,623, 3,969,071, 4,021,591, 4,058,644, 4,171,202, 4,171,230, 4,205,991 and 4,278,434. This technique can also be used to dye substrates other than fabric as shown in U.S. Pat. Nos. 3,239,366, 3,508,492 and 4,059,471.
In dry transfer printing, designs are printed using a variety of techniques such as letterpress, flexographic, gravure, rotary screen or offset lithography. The carrier onto which the designs are printed must be a sheet of a heat resistant material. Paper or metal foils are the preferred carriers used in virtually all applications because they are heat resistant at the temperatures between about 150.degree. F. and 500.degree. F., and generally between about 350.degree. F. and 450.degree. F., and for the periods of time from a few seconds to a few minutes required to vaporize the sublimable dyestuff in the ink. All other carrier materials which are not heat resistant, such as plastic films, would become deformed or destroyed or would be dyed themselves at such elevated temperatures. The inks containing the sublimable dyes also contain resins and other binder components such as surface active agents and viscosity modifiers. These binder components function to hold the dyestuff onto the paper carrier and remain on the carrier following sublimation of the dyestuff. Therefore, these binder components cannot interfere with the partial sublimation and transfer of the vaporized dyestuff from the ink to the fabric surface, nor can they contain any wax or other meltable or thermoplastic components which would interfere with the transfer process. Under the application of heat and pressure, these meltable components would smear from one surface to the other producing a diffused or discontinuous image caused by a spreading of the ink.
Transfer printing techniques are as unsuitable as the other large scale commercial printing methods for printing directly on fabric labels. The transfer papers must first be printed with the characters or design which is to be transferred to the labels. Preprinting is usually done off site in large volume to be economic. In order to ensure that each garment will receive the proper labelling which is required, a large inventory of different labels must be maintained. If the printing were to be done in-plant, an equally large inventory of different preprinted transfer papers would be required. Moreover, the printing rollers are very expensive and slow to produce.
As described above, the most economical and efficient method of in-plant printing involves hot stamping technology utilizing carrier supported inks. Fabric labels printed using carrier supported inks cannot withstand extremely corrosive and abrasive laundering conditions to the same degree as labels in which the fabric has been dyed. Transfer papers, such as those employed in dry transfer printing techniques to dye fabric with preprinted designs, however, could not be employed in hot stamping printing systems that presently employ carrier supported inks. Because transfer papers must be sufficiently heat resistant to withstand the relatively high temperatures and long dwell times required to vaporize the dyestuffs printed on the papers to cause them to transfer to the fabric substrates, they simply would be inoperative in hot stamping equipment in which the printing elements are at lower temperatures and the dwell times are very short. The heat resistant paper would prevent the dyestuff from reaching its sublimation temperature and transferring to the fabric substrate. Moreover, the ink formulation used to print transfer papers is designed to remain on the paper during sublimation and transfer of the dyestuff to the fabric and therefore, would be inoperative under the conditions employed in hot stamping equipment. If a transfer paper were to be made with a less heat resistant carrier material, then the temperatures and dwell times required to vaporize and transfer the dyestuff would destroy or deform the carrier and cause a poor image to be transferred to the fabric substrate.